Electrostatic spray heads



April 1963 H. v. SCHWEITZER ETAL 3,085,749

ELECTROSTATIC SPRAY HEADS 3 Sheets-Sheet 1 Filed Aug. 8, 1960 INVENTOR.

April 16, 1963 H. v. SCHWEITZER ETAL 3,085,749

ELECTROSTATIC SPRAY HEADS Filed Aug. 8, 1960 5 Sheets-Sheet 2 FIG. l2

INVENTORS RICHARD J. VERBA HOWARD V. SCHWEITZER 2 7. 4% w gmdw ATTORNEYSApril 16, 1963 H. v. SCHWEITZER ETAL 3,085,749

ELECTROSTATIC SPRAY HEADS Filed Aug. 8, 1960 3 Sheets-Sheet 3 INVENTORSRICHARD J. VERBA HOWARD v. SCHWEITZER 64 mv mw ATTORNEYS btate Thisinvention relates to an improvement in electrostatic spray heads and,more particularly, to spray heads which improve the distribution in andconfinement to an established electrostatic field of particles ordroplets dispersed in said field. The improved heads are especiallyuseful in equipment for coating paints (including enamels, lacquers,ceramic coatings, etc.) on an intended receiving surface. Thisapplication is a continuation-in-part of our application forElectrostatic Spray Head, Serial No. 599,530, filed July 23, 1956, nowabandoned, without any abandonment of the inventions disclosed and/orclaimed therein, in favor of this present application.

Electrostatic spray equipment has long been used for applying coatingcompositions to objects; for spinning collodion, lacquers, etc., intosynthetic fiber staples; for applying discrete solid particles, such asgrit and flock to adhesive surfaces; for concentrating liquids such asmilk, etc.; and/or the like. The fundamental principle in any suchoperation is that, in discharging material (such as a liquid or afluid-like mass of solid particles) into an electrostatic fieldextending to a receiving surface, electrostatic charges are presumablyimposed on the material. These charges cause a further dispersion of thematerial (atomization or attenuation in the case of liquids and specialorientation in the case of discrete solid particles) due to therepelling effect of charges of the same polarity on each particle (solidor liquid) of the material and, simultaneously, attract the particles toa receiving surface. The receiving surface either has a charge actuallyor effectively opposite in polarity to the particles. If the receivingsurface is not actually of a polarity opposite to the charge on theparticles, an effectively opposite charge may be imposed in severalways. For example, the receiving surface may be of the same polarity butof a lesser potential than the source in the electrostatic field fromwhich the particles received their charge. Also, the receiving surfaceitself may be non-conductive and carry no charge but simply beinterposed in the path of travel of the charged particles in theelectrostatic field toward a conductive surface or point, whereby thereceiving surface intercepts (and usually holds) the charged particles.

Whether liquid particles dispersed in the abovedescribed system arestill liquid when received upon the receiving surface, a fine powder,attenuated fibers, or merely a concentrate from which a volatilecomponent evaporated in the course of travel between the spray head andreceiving surface depends upon the physical characteristics of theliquid and the condition and type of atmosphere through which the liquidpasses between the head and receiving surface. Thus, if a relativelylong liquid having a volatile solvent, such as collodion, is sprayed anda suitable distance is provided, it will form clusters of fibers on thereceiving surface, which fibers may then be removed and spun intotextile yarns, for example. If the liquid is short and has a highlyvolatile solvent, as in the case of certain wax solutions, the sprayedmaterial may be collected as a fine powder on the receiving surface. Ifthe liquid be a paint or lacquer having at least some relatively lowervolatile solvents in its composition, it usually will be deposited inliquid form on the receiving surface and, depending atent O upon theamount deposited on a given area, will form a coating film. If theliquid should be a so-called hot melt paint, Whether the receivingsurface will receive a powder or film-forming coating will depend on thetemperature of the atmosphere and the temperature of the paint beforeatomization. Also, depending on the atmospheric temperature and/orpressure, milk, for example, may be either concentrated or powdered,depending on the amount of liquid evaporated during atomization.

Other conditions and characteristics which appear to afiFect thecondition of particles (sprayed on in the form of a liquid from anatomizing head) when received on a receiving surface include the time oftravel from the atomizing head to the receiving surface and thedielectric characteristics of the sprayed liquid (which may includepigmented liquids). Other conditions being equal, a liquid having avolatile solvent will be received on a receiving surface in a drier,more nearly powdered form if it requires a greater time to travel fromthe atomizing head to the receiving surface than the same liquidrequiring a short time of travel. The dielectric strength of a liquidbeing electrostatically sprayed affects the condition in which theliquid is received on the receivstrength appear to carry lesser chargeswhen atomized than liquids of lower dielectric strength, and thus appearto depend more upon the mechanical atomization effected by anelectrostatically charged spray head than liquids which have a lowerdielectric strength. For example, assuming a lacquer of relatively highdielectric strength (e.g., a lacquer having a phenolic or polyesterresin base) and a lacquer of relatively low dielectric strength (e.g., alacquer having a nitrocellulose base) are otherwise equal in viscosity,surface tension, proportion of volatile solvents of substantially equalvolatility, conductivity, etc., are sprayed under equal conditions, thelacquer of lower dielectric strength will be received on the receivingsurface in a drier condition than the lacquer of higher dielectricstrength. The explanation for this re sult appears to be that the moreconductive lacquer is more finely atomized by the electrostatic forcesacting on the particles mechanically atomized by the spray head. Suchfiner atomization increases the surface presented for evaporation duringthe time of travel from the spray head to the receiving surface.

Those skilled in the art employing spray heads made according to thisinvention in electrostatic spraying must, for best results, bear theforegoing general conditions and requirements in mind in formulating theliquid or establishing the line of travel between the spray head andreceiving surface if the heads are to be used for coating, evaporating,fiber or powder production, or like uses.

Regardless of the use to which an electrostaticallycharged spray is put,a principal object of any spray head is to impart an electrostaticcharge to each particle during (and preferably at the commencement of)the travel from the spray head to the receiving surface. As betweennozzles, air spray guns, centrifugal devices, traveling belts, etc.,centrifugal heads have generally been the most convenient, especially inthe application of coatings, to which this invention is particularlydirected. The centrifugal device, heretofore a circular disk or cup,will, even at rela tively slow speeds, do a fair job of atomization in aplane perpendicular to the axis of rotation without an electrostaticfield being imposed. When an electrostatic field of high potential isimposed between the disk or cup and the receiving surface to be coated(which is usually located in front of the disk or cup), the chargeimparted to the liquid particles when they leave the spray head willtend to atomize the particles further or, at least, atomize them to amore uniform size and impart to them a commg surface in this fashion:Liquids of high dielectric ponent of movement toward the intendedreceiving surface or object. In fact, because the component of movementof the spray particles from a centrifugal head to the object usually canbe attributable largely if not solely to the electrostatic field inwhich the particle moves, it has long been the goal of the art toprovide a head from which the particles would move toward the objectonly along the lines of electrostatic force between the head and theobject and, thus, the particles would deposit only on the object.Thereby complete utilization of the paint would be obtained and noover-spray would result. Unfortunately, in any atomizing head developedso far, the combination of mechanical and electrostatic forces have beeninsufficient to handle a practical workable volume of paint and, at thesame time, so thoroughly atomize the paint that the electrostatic forcescompletely overcome the centrifugal impetus imparted to the sprayparticles by the spray head. As a consequence, some particles may have amomentum which carries them outside the effect of the electrostaticfield and so-called over-spraying can result. An object of thisinvention is to provide a spray head which thoroughly atomizes paint andlike liquids so that the particles are relatively mass-less andover-spraying is minimized.

Another ditficulty with prior art centrifugal electro static spray headsis that the heads have been designed to provide a circular dischargeedge substantially concentric with the axis of rotation and frequentlyfairly intricate or elaborate means have had to be employed todistribute the paint more or less evenly to this circular dischargeedge. Such spray heads and the means for distributing paint to thecircular edges are frequently difficult to clean or change. When achange in the color or type of coating composition is required duringproduction, excessive down-time to change or clean the heads may renderthe whole coating set-up uneconomical or undesirably inconvenient. Anobject of this invention is to provide a spray head which can be easilycleaned or changed with a minimum of down-time.

Other objects and advantages of this invention will be apparent from thefollowing specification, claims, and drawings, in which:

FIGURE 1 is a diagrammatic showing of an electrostatic coating equipmentset-up suitable for improved centrifugal spray heads made according tothis invention.

FIGURE 2 is a front elevation of one embodiment of a spray head madeaccording to this invention.

FIGURE 3 is a cross-Section taken along the line 33 of FIGURE 2.

FIGURE 4 is a front elevation of another embodiment of a spray head madeaccording to this invention.

FIGURE 5 is a cross-section taken along the line 5-5 of FIGURE 4.

FIGURE 6 is a front elevation of still another embodiment of thisinvention.

FIGURE 7 is a cross-section taken along the line 77 of FIGURE 6.

FIGURE 8 is a fragmentary detail of a modification of the embodimentshown in FIGURE 6.

FIGURE 9 is a detail, corresponding to FIGURE 8, of a furthermodification of the embodiment shown in FIGURE 8.

FIGURE 10 is a detail, in cross-section, of a still further modificationof the embodiment shown in FIGURE 8.

FIGURE 11 is a cross-section of a further embodiment.

FIGURE 12 is an enlarged detail plan view taken from the line 1212 ofFIGURE 11.

FIGURE 13 is a cross-section of a still further embodiment.

FIGURE 14 is an enlarged detail plan View taken from line 14-14 ofFIGURE 13.

FIGURE 15 is a front elevation of an additional embodiment.

FIGURE 16 is an enlarged cross-section taken along line Iii-4.6 ofFIGURE 15.

- tration.

FIGURE 17 is a front elevation of still an additional embodiment.

FIGURE 18 is a detail cross-section of the embodimen shown in FIGURE 17taken along line 13-43 of FIG- URE 17.

As indicated above, spray heads made according to this invention aresuitable for use in the various applications for electrostaticdispersion, but are particularly suited for electrostatic coatingoperations. FIGURE I shows dia grammatically a typical coating set-upsuitable for spray heads made according to this invention. Variousfieldshaping electrodes, anti-sparking and current-limiting protectivedevices for the high-voltage potential, and the like may be employed butare omitted for simplicity of illus- Likewise, one may employ anysuitable type of mechanism for moving the spray head relative to theobject during operation and for controlling the movement of the objectduring operation; such mechanisms are similarly omitted for simplicityof illustration.

As indicated in FIGURE 1, the spray head H is r0- tated by the shaft Smounted in a journal housing I carried by the stand for the motor M. Thejournal housing I is usually of non-conductive material or at least soinsulated that a high potential insulation exists in the journalstructure between the shaft S and the motor support. Similarly, theshaft is connected to the motor through an insulating coupling I toprevent the high potential applied to the head H from shorting back tothe motor or stand, which, as indicated, is usually grounded for safetyto operating personnel. The actual head H is connected to the shaft Sthrough a chuck C, which is preferably mounted exteriorly of the journalhousing for simplification of access in changing heads if and whenrequired. The high electrical potential for the head is supplied from asource of high voltage HV, usually through a simple slip-ring R. Paintis supplied to the head from a nozzle N carried by the journal housingand terminating just short of the disk portion of the head H. The nozzleN is usually of non-conductive material, such as glass, and is connectedto a paint supply PS through a suitable tubing T. In most cases thepaint or like coating composition to be sprayed is sufficientlynon-conductive, as is the tubing T, so that the pressure tank, controls,and the like at the paint supply PS may be grounded without shorting outthe high potential supplied to head H. If, however, the coatingcomposition itself is relatively conductive, due to conductive pigmentsor a relatively conductive vehicle or solvent in the coatingcomposition, steps should be taken to insulate the entire paint supplysystem, from the nozzle N to the equipment in the paint supply equipmentPS, in order to avoid shorting out the head H; in such cases the systemis also usually suitably shielded to protect personnel.

The object O to be coated is suitably supported in front of the head H.In most instances the object is one of many carried by a suitableconveyor whose path of travel before the head H is in a plane parallelto the face of the head H, i.e., in a plane perpendicular to an extendedaxis of rotation of the head H and shaft S. The object O, which may beof a wide variety of con figurations, is actually or effectively at apolarity opposite to that of the head H.

In operation, the motor M rotates the head H, paint is supplied to thehead H from the nozzle N, and an electrostatic field is established bythe source of high potential between the head H and the object 0, saidfield F being indicated by the broken light lines from H to 0. If thehigh potential to the head H is shut off, the paint will be atomized toa substantial degree by being thrown centrifugally off the outer edge ofthe head H in the plane indicated in FIGURE 1 by the arrows Pl extendingaway from the head; there will be no substantial components of travel ofthe paint particles so atomized r toward the object O. A slightcomponent of travel toward the object 0 may be imparted to the paintparticles by dishing or cupping discharge points of the head H towardthe object 0 so that the paint is carried by the head H axially towardthe object as it also moves radially outwardly from the point ofdischarge onto the head H from the paint supply outlet, but the impetusimparted to paint particles by such axial travel is usually negligiblecompared to the impetus imparted by the centrifugal force moving thepaint radially. In fact, since an object and purpose of the field F isto pull the paint particles discharged centrifugally by the head H fromthe plane PP (in which the particles would normally travel except forthe influence of the field F) and converge them toward the object 0, itis preferable practice to rotate the head no more rapidly than necessaryto move the paint to the edge of the head H and there break the paintinto droplets sufliciently small to permit the electrostatic field tofurther atomize the particles into relatively weightless particles whichcan be carried by the electrostatic charges thereon toward the object;otherwise, a substantial amount of the paint might be throwncentrifugally out of the field F. While the rotational speed of the headwill vary according to many factors (such as the potential across theelectrostatic field F, the viscosity of the paint to be sprayed, andother factors including those mentioned in the preamble of thisspecification), with a potential gradient of approximately 10,000 voltsper inch of minimum spacing from the head to the object the rotationalspeeds are usually in the range of 400 to 4,000 r.p.m.

In centrifugal electrostatic spray heads employed heretofore, thedischarge edge of the heads, whether simple disks or cups, haveuniformly been circular and concentric with the axis of rotation of thehead, and careful eifort has been made to secure distribution of uniformamounts of the paint to equal integers of the circular edge. To theextent that this prior art construction and distribution has beenrationalized at all, it has been based on the mathematical assumptionthat the electrostatic charges accumulated on the edge of the head andimparted to the droplets of paint discharged therefrom would distributethemselves equally around the periphery of the circular edge. Thus,presumably, droplets of uniform volume would be uniformly thrown off theedge about the periphery, carrying uniform charges per droplet, whichcharges, being of the same polarity, were repulsive to each other andwould thereby further atomize the droplets to fog-like particles whichwould be drawn by the field to the surface of the object to be coated.

The improvement effected by this invention stems from a rejection of theforegoing premise assumed by the prior art, namely, that electrostaticcharges would necessarily distribute themselves uniformly about theperiphery of even a circular discharge disk and, thus, that a circulardischarge edge for the disk would be the optimum configuration for thatedge and that paint .should be fed uniformly to integers of theperiphery of the disk. Such prior art spray disks discharge droplets ofunequal size, especially when any substantial and economical volume ofpaint is supplied to the disk, resulting in the discharge of largerdroplets not completely controlled by the field in their travel to theobject and consequent overspray. Further, under the dynamic conditionsof paint flow from the heads, it is possible, even on a circular disk,to observe that the paint evidently prefers to leave the discharge edgeat fairly evenly spaced points. When spraying a coating compositioncomprising a solution of resinous material in a highly volatile solvent,such as an ether collodion, these preferred points of discharge will berevealed by deposits of the resinous material in the form of teeth orpins on the discharge edge, such teeth or pins pointing to the object.This invention takes advantage of and emphasizes this natural tendencyby employing a non-circular disk having a plurality of apices or pointsat the periphery of the disk. The apices are preferably spaced at equalangles with respect to each other and the number of apices is preferablyless than the number of points from which the paint would tend todischarge if the periphery of the disk were circular. Thus, when a paintwould tend to discharge from about thirty to forty points about acircular disk, improved atomization appears to be obtained and a greatervolume of paint may be handled by a disk having as few as three or fourapices from which the substantial majority of the paint will bedischarged.

Further, the paint atomized from a head made according to this inventionappears to be more uniformly dispersed and the inner diameter of thecone of paint appears to be less. That is to say, when a circular priorart head is used, the paint is discharged therefrom in the form of ahollow cone; when the object is a large stationary sheet, the paintfalls in a circular path, the inner diameter of which may be as much astwo-thirds of its outer diameter. Paint discharged from a head madeaccording to this invention will likewise assume the form of a hollowcone, but the inner diameter should be smaller and the outer diameterlarger. Thus, where a plurality of staggered prior art spray heads maybe required to cover objects passed before them, a single head or alesser number of heads made according to this invention may be employed.

A rationalization of the operation of spray heads made according to thisinvention is that the several apices at the periphery tend to providepoints when the charges induced by the field tend to accummulate ingreater concentration or potential than if a circular periphery wereemployed. At the same time, although paint is discharged onto the diskuniformly about a path concentric with the axis of rotation, it willtend or may be made to flow primarily from the apices. Presumably due tothe high accumulation or potential of charges adjacent the apices, paintwill tend to atomize under the influence of the electrostatic field evenbefore it reaches the points of the apices and be drawn to the object bythe electrostatic field. Such paint as leaves the apices at these pointsis apparently now completely atomized to such relatively mass-lessparticles that very few leave the influence of the field in the travelfrom the head to the object. Other rationalizations and explanations ofthe operation of spray heads made according to this invention may bedeveloped as further knowledge and understanding in the art develops.

FIGURES 2 and 3 illustrate a very simple embodiment of a spray head madeaccording to this invention. The head comprises a square disk 10 havinga central perpendicular shank 11. The end of the shank 11 is suitablynotched to be received in a chuck C of the setup shown in FIGURE 1. Thereverse surface of the disk 10 is preferably slightly dished to improvedistribution of paint to the edges. While not necessary to successfuloperation of the disk 10, the reverse surface may also be grooved toguide the flow of paint more generally toward the points of the apices12, which points are provided by the four corners of the square disk. Toprovide a sharp edge around the periphery and sharp points for theapices 12, a bevel 13 is ground on the margins of the disk 10. For somereason which is not understood, operation appears to be improved if theface 14 of the disk 10 within the marginal bevels is slightly convex.

With the disk 10 mounted in the chuck C of the set-up shown in FIGURE 1,paint is discharged at a metered rate onto the reverse face of the disk10 from the nozzle N. The paint is carried centrifugally toward theapices 12, from the points and adjacent edges of which the paint isatomized and carried by the electrostatic field to the surface of theobject to be painted.

FIGURES 4 and 5 illustrate another embodiment designed to permit paintto discharge from the face of the apices as well as the edges. In thisembodiment the head comprises a disk 101, which, in this particularinstance, is triangular. The disk 1111 has a central opening Hi2 throughwhich the shank 116 extends to an outwardly convex cover 111, which isunited with the disk 161 at the edges of the disk intermediate theapices 112. Adjacent the apices the cover 111 is relieved to provideopenings 113 through which paint may flow to the surface of the apices112; these openings 11?: may further serve as orifices to meter theamount of paint fed to the apices, if desired. In this particularinstance the shank 1121 is drilled to receive the shaft S, to whichshaft the shank 11% can be readily attached and detached by its collarand setscrew 115.

As should be apparent, in operation paint is fed from the nozzle Nthrough the opening 1132 into the chamber provided by the disk 1131 andthe cover 111. From this chamber the paint flows centrifugally throughthe openings 113 to the apices 112, from whence it is atomized andcarried by the field F to the object to be coated.

It is not necessary that the spray head apices be the corner areas ofregular polygons, as shown in FIGURES 2 to 5. They may be the points ofstars or even needles, as in the head 28% shown in FIGURES 6 and 7. InFIG- URES 6 and 7, the shank 210 carries a boss 201 which, in turn,carries a plurality of equally angularly spaced radial needles 212. Inthis particular instance the needles are also inclined toward theobject. The needles 212 extend through orifices 213 in a cover 211carried by the shank 210 ahead of the boss 201. The cover 211 has itsedges spun inwardly to provide a rearward opening 2112. Paint fed fromthe nozzle N into the chamber provided by the cover 211 flows radiallyoutwardly through the orifices 213 onto the needles 212, from whence itis atomized and carried by the electrostatic field to the object to becoated.

It is not necessary that apices from whence the paint is atomized extendradially from the shank but only that a suitable path or surface beprovided so that some of the paint may flow to the point of an apex.Thus, as indicated in FIGURE 8, the needles of the embodiment shown inFIGURES 6 and 7 may be curved, preferably away from the direction inwhich the head is rotated. In this particular instance the curvature issuch that the needle 212a provides a surface to which a droplet of paintmight cling as the droplet would move radially outwardly under influenceof centrifugal force. The curvature in this instance is determined bythe rotational speed of the head and the velocity of a paint droplet asit would leave the orifice 213 of the head 260. In the operation of thehead 200 at capacity, a substantial portion of paint may be observed toleave the needles 212 long before some of the paint reaches the pointsof these straight needles, just as in the head paint Will be observed toatomize from all portions of the periphery. Some of such atomization maybe attributable solely to centrifugal force, rather than theelectrostatic field. With the somewhat longer curved needles 212a,atomization does appear to be improved and over-spray somewhat reduced.Whether this apparently improved atomization is due to the fact thatdroplets making up the streams of paint carried by the curved needlesare given a greater opportunity to accumulate electrostatic charges oris simply due to the fact that the droplets are carried to a point wherethey will be more finely atomized by centrifugal force appears to be amatter of academic theory as far as results are concerned.

Each apex from which paint is centrifugally discharged andsimultaneously electrostatically sprayed may have one or moresub-apices. Thus, the embodiment shown in FIGURE 7 may be furthermodified by having the radial needles shown therein suitably branched,either in a plane transverse to the axis of rotation of the head as in aplane radial to said axis or in a plane or planes skew to said axis.FIGURE 9 shows a needle 212 having branched apices 1212b in a planetransverse to the axis of rotation. FIGURE 10 shows a needle 212 havingbranched apices 212c in a plane radial to the axis of rotation. Inaddition to the branching of the needles 212, as shown in FIG- URES 9and 10, the needles 212 may be otherwise varied. For example, instead ofbeing circular in cross-section, they may be fluted, rectangular, orotherwise modified in cross-section; if non-circular, elements on thesurface may be parallel to the needle axis or twisted. The orifices 213,which seem to have the effect of metering the paint which flows to theneedle, may also be varied from a circular form. Thus, the inwardlydirected cusps of a fluted orifice can hold a needle 212 substantiallycentered within the orifice opening. Other cross-sections of the orificewhich are not concentric or equidistant at all points from correspondingpoints on the needle may, under given conditions, aid in controlling thesize or rate of droplets discharged from the needles.

Another embodiment employing branched apices is shown in FIGURES l1 and12. A head 300 is mounted in the shank 310 and comprises a front cover311 having a spun-over edge 315 to provide an opening 302 into whichpaint may be fed from a feed pipe N corresponding to the nozzle N shownin FIGURES 1, 3, 5, and 7. Orifices 313 are punched in the periphery ofthe cover 311 to provide multiple apices 312. The apices 312 not onlyserve to break up the paint discharged through the orifice 313 but alsoserve as discharge points for electrostatically atomizing the paintpassing over them.

The more nearly the apices from which the paint is discharged approachesa crown having its points directed toward the receiving surface, themore effective the atomization appears to become, provided the apicesalso provide points which efiect a locus for concentrated electrostaticcharges. In accordance with this crowning principle, a furtherembodiment, constituting a variation of the embodiment shown in FIGURES11 and 12, has been originated. Thus, as shown in FIGURES 13 and 14, ahead 4610 is mounted on a shank 4161 and comprises a cover 4-11 having aspun-over edge 4-15 to provide a rear opening 4&2 into which paint maybe fed through a feed pipe N. The cover 411 is provided with orifices413 opening into conical conduits 414 inclined toward the receivingsurface. The open apex of each conical conduit 414 is preferablyserrated to provide a plurality of sub-apices 412. These sub-apices, asthe apices 312, serve to break up the paint discharged through theconduits and discharge points for electrostatically charging the paintpassing over them.

The apices from which paint is discharged does not always need to becrowned toward the receiving surface and good results can be obtainedwhen the apices lie generally in a plane perpendicular to the axis ofrotation of the spray head. Thus, as shown in FIGURES l5 and 16, ahollow head 501), having an opening 502 for receiving paint to beatomized, is shaped to provide a plurality of hollow, radially extendingpins 514 terminating in apices 512. The pins are preferably beveled atthe apices to provide discharge edges, as shown in FIGURE 16. As in theother embodiments, paint discharged from the apices 512 appears to besimultaneously mechanically and electrostatically atomized.

It is to be noted that in the various embodiments shown in FIGURES 4 to16, the head, between the front cover and the paint receiving openingand radially beyond the opening, affords a capacity or volume receivingthe paint discharged from the nozzle N or feed pipe N. This volume orcapacity is advantageous in that it provides a reservoir for paint to bedischarged at a substantially constant rate from each apex throughout arevolution of the head. Accordingly, in the embodiments having a paintreservoir, it is generally immaterial where the paint nozzle or feedpipe be located so long as paint is discharged into the head at a rateapproximately equal to the rate of discharge of paint from the head. Incontrast, in the embodiment shown in FIGURES 2 and 3, the paint isgenerally discharged into the back of the head above the stank in orderthat the centrifugal force of the head may distribute the paint to theapices before the paint falls off the head under the influence ofgravity; as a consequence, paint will tend to discharge within a limitedarc, and the apices will tend to be starved for paint to atomize duringthe balance of its revolution. Without a reservoir, therefore, thevolume of paint which can be handled by a spray head may be limited tosubstantially the amount of paint which can be atomized by the apices asthey pass through the aforesaid arc. While a plurality of nozzles orfeed pipes may be employed in embodiments affording no paint reservoirin order to minimize variations in discharge from each apex, wherereservoir means are provided, a single nozzle or feed pipe can besufficient to provide a substantially constant discharge and a greatertotal volume of discharged paint for the atomizing head.

Another feature of the embodiments discharging the paint throughconduits or orifices is that the passageways may be tapered or otherwiseconfigured to accelerate the velocity of the paint toward the apices.Such acceleration appears to aid in atomization.

While embodiments employing accelerating conduits and/or orifices incombination with a reservoir are often preferred, embodiments Withoutsuch features are often very satisfactory. In addition to such a simplefeed embodiment as shown in FIGURES 2 and 3, another such embodiment isshown in FIGURES 17 and 18 in which the head 600 comprises the shank 610carrying a disc 601 provided on its periphery with vanes 614, each vanebeing twisted from a plane transverse to the axis of rotation of thehead toward a plane radial thereto. Paint discharged from a suitablenozzle N into the rotating vanes or onto the disc 601 will beaccelerated toward the tips 612 of the vanes, which are twisted towardthe receiving surface. Thus, for the purpose of this embodiment, theforward vane tips 612 constitute the apices from or near which the paintis discharged forwardly of the head after it has been discharged ontothe back of the head.

In all of the foregoing specific and illustrative embodiments, it shouldbe noted that the head may be removed from the driving shaft withouthaving to disconnect it from the paint supply system and, further, thatthe construction permits ready cleaning of the head by simply dousing itin solvent. To change color or to apply a different type of coatingcomposition, a head is simply unchucked and replaced by a cleanduplicate or is quickly cleaned and replaced; the new color or type ofpaint is supplied from a separate paint supply system whose nozzle isclipped or otherwise secured to the journal housing.

The size of any given head and the number of apices employed depends ona number of factors, such as the area which the head is expected tocoat, the amount of paint to be atomized, the potential of the fieldbetween the head and the object, the shaping of the field as objects areconveyed past the head, and the like. In general, the number of apicesshould approach the minimum number which can handle the volume of paintto be atomized without creation of large droplets, rather than so manyapices that the disk becomes saw-toothed and paint tends to fill thegaps between the teeth; if there is a tendency of the paint to run tothe gaps between the apices, dams, grooves, or covers are preferablyemployed to direct the flow toward the apices.

In the operation of the several embodiments, a substantial ionization ofthe atmosphere adjacent the apices appears to be maintained. While thisionization is presumably due to the concentration of static charges atthe apices under the influence of the potential between the head and thereceiving surface, but, in attempting to explain the principle ofoperation of this invention, it is not entirely clear what part suchionization may play in effecting the atomization and charging of theparticles of paint discharged from the head. An assumption that theionized atmosphere aids in breaking up droplets centrifugally dischargedfrom the apices may be combined with an assumption that the chargedparticles discharged centrifugally from the spray head maintainionization of the atmosphere adjacent the head.

In most of the embodiments illustrated, it is to be noted that beforethe paint reaches or can reach the point of an apex, it is given anappreciable distance of travel over an external surface adjacent thepoint of the apex. This appears to be desirable in order to give thepaint an opportunity to pick up static charges as it approaches theapex. The embodiment shown in FIGS. 15 and 16, however, is one exceptionand may be explained on the assumption that, at the instant a dropletleaves the beveled end of a hollow pin 512, the droplet itselfmomentarily becomes the point of the apex and, when the droplet breaksaway from the pin, at a succeeding instant, it carries with the chargeconcentrated at the apex.

Whether any one of the foregoing attempted explanations of theprinciples of operation of embodiments of this invention are laterestablished as the correct one or entirely different theories areadvanced, it is to be understood that other and different variations ofthis invention may be made by those Skilled in the art without departingfrom the spirit and scope of the invention as defined in the followingclaims.

What is claimed is:

1. An electrostatic spray system in which atomized particles are movedat least in part by electrostatic field forces from a head to areceiving surface, a paint feeding means, a rotatable head comprising ashank, a disk structure carried by said shank and rotatable about theaxis of said shank, said disk structure carrying a plurality of apiceson its periphery and having surfaces connected with said shank forrotation thereby and extending toward said apices permitting apicadmovement of fluent material discharged from said paint feeding meansonto a portion of said surface located inwardly of said apices.

2. A spray head for electrostatic spray systems comprising a shaft forrotating said head, means for creating a potential between said head andan object to be coated, and means for conveying a coating composition tosaid head, said head comprising a disk structure, a plurality of spacedapices in the periphery of said disk structure, a surface in said diskstructure for receiving coating composition and conveying said coatingcomposition to said apices, and means to mount said disk structure onsaid shaft, whereby said apices will be rotated in a plane perpendicularto the axis of rotation of said disk structure, and said surface forreceiving coating composition being located adjacent said mounting meansand positioned ad jacent the means for conveying a coating compositionto said head, said surface being positioned by said shaft be tween saidcoating composition conveying means and said object to be coated.

3. An electrostatic discharge head comprising a disk, means for mountingsaid disk on a shaft for rotation in at least one plane perpendicular tothe axis of rotation of said shaft, said disk having a plurality ofapices in the periphery thereof and at least one surface permitting flowto said apices of coating composition discharged onto said disk betweensaid periphery and the axis of rotation thereof, the surface of saiddisk receiving said coating composition being on the shaft side of saiddisk.

4. An electrostatic spray discharge head as defined in claim 3 in whichsaid disk has a polygonal configuration, the outer corners of whichconstitute said apices, and a face and reverse surface converging to asingle edge at and adjacent the points of said apices.

5. An electrostatic spray discharge head as defined in claim 4 in whichthe face of said disk is generally convex and the reverse surface isgenerally concave.

6. An electrostatic spray discharge head as defined in claim 3, in whichsaid disk is provided with at least one opening in the reverse surfacethereof, said opening permitting the flow from the reverse surface tothe face of said disk adjacent the apices thereof of coating composition supplied to the reverse of said disk.

7. An electrostatic spray discharge head as defined in claim 6 whereinsaid opening opens into a cavity within said head, said cavity havingoutlets therefrom, each outlet being more adjacent an apex, in a radialdirection, than said opening, whereby said cavity may serve as areservoir for a coating composition fed into said opening anddistributed to said outlets as said head is rotated, said cavity beingof a suflicient volume and said outlets being sufficiently constrictedto maintain a substantially constant flow of said coating compositionfrom each outlet to its adjacent apex as said head is rotated.

8. An electrostatic spray discharge head as defined in claim 3 in whichthe opening in said apexed disk is a central opening through which theaxis of rotation of said shaft extends and including a cover located onthe face of said disk over said opening and providing openings betweenthe face of said disk and cover adjacent said apices, whereby, when saiddisk is rotated, paint discharged into said openings will be distributedby said cover toward said apices and on the face of said disk.

9. An electrostatic spray discharge head as defined in claim 3 in whichsaid apexed disk comprises a plurality of members having needle-shapedextremities extending in a direction havng at least a component radialto said shaft and means directing, when said disk is rotated, the flowof a coating composition discharged onto said disk toward theextremities of said members.

10. An electrostatic spray discharge head as defined in claim 9 in whichsaid flow-directing means directs the flow of a coating composition overthe external surfaces of said members adjacent their extremities as Wellas toward said extremities.

11. An electrostatic spray discharge head as defined in claim 10 inwhich said members beyond said flow-directing means are curved toprovide a surface to which a coating composition may cling while itmoves on the extremities of said members in a substantially straightlinear path extending radially from the axis of rotation.

12. An electrostatic spray discharge head as defined in claim 3 in whichat least one of said apices comprises a plurality of associated pointedmembers.

13. An electrostatic spray discharge head as defined in claim 3 in whichsaid disk comprises a hollow member defining a cavity substantiallysymmetrical about the axis of rotation, said member having an openingconcentric with said axis, whereby a liquid coating composition may befed into said cavity through said opening and distributed centrifugallyinto the portions of the cavity located radially outward of said openingwhen said disk is rotated, and said member having outlets from saidcavity, said outlets being located in the portion radially outward ofsaid opening and leading to the apices of said disk.

14. An electrostatic spray discharge head as defined in claim 13 inwhich said passageways lead to a plurality of teeth constituting an apexof said disk.

15. An electrostatic spray discharge head as defined in claim 13 inwhich said passageways are defined by tubular members whose tipsconstitute the apices of the disk.

16. An electrostatic spray discharge head as defined in claim 15 inwhich said tubular members extend in a direction both axially andradially divergent from said axis of rotation, whereby the tubularmembers may impart to the coating composition moving therethrough animpetus in the direction of the object to be coated.

17. An electrostatic spray discharge head as defined in claim 15 inwhich said tubular members are provided with a varying cross-section.

18. An electrostatic spray discharge head as defined in claim 3 in whichsaid disk is provided with radially extending, angularly spaced vanemembers, the radially outer tips of which vane members constitute theapices of said disk.

19. An electrostatic spray discharge head as defined in claim 18 inwhich said radially outer tips of said vane members are twisted axiallyforward of said shaft.

References Cited in the file of this patent UNITED STATES PATENTS1,753,019 Page Apr. 1, 1930 2,685,536 Starkey et al. Aug. 3, 19542,893,893 Crouse July 7, 1959 2,961,581 Grave et al Nov. 22, 1960

1. AN ELECTROSTATIC SPRAY SYSTEM IN WHICH ATOMIZED PARTICLES ARE MOVEDAT LEAST IN PART BY ELECTROSTATIC FIELD FORCES FROM A HEAD TO ARECEIVING SURFACE, A PAINT FEEDING MEANS, A ROTATABLE HEAD COMPRISING ASHANK, A DISK STRUCTURE CARRIED BY SAID SHANK AND ROTATABLE ABOUT THEAXIS OF SAID SHANK, SAID DISK STRUCTURE CARRYING A PLURALITY OF APICESON ITS PERIPHERY AND HAVING SURFACE CONNECTED WITH SAID SHANK FORROTATION THEREBY AND EXTENDING TOWARD SAID APICES PERMITTING APICADMOVEMENT OF FLUENT MATERIAL DISCHARGED FROM SAID PAINT FEEDING MEANSONTO A PORTION OF SAID SURFACE LOCATED INWARDLY OF SAID APICES.